Maintaining electronic devices, such as microprocessors in data centers, within safe operating temperature ranges is a challenging problem that is only increasing in importance and difficulty as semiconductor technology continues to progress and as popularity of cloud storage continues to grow. State of the art microprocessors can easily produce more than 40 watts of thermal energy per square centimeter of microchip surface area, and power electronics can attain heat densities three times that level.
In addition to the need to manage such high heat densities, there is also a need to do so in an efficient manner, in terms of energy expended to cool these heat sources. According to the Department of Energy, nearly three percent of electricity used in the United States is devoted to powering data centers or computer facilities. Approximately half of this electricity goes toward power conditioning and cooling. Increasing the efficiency of cooling systems would lead to dramatic savings in energy nationwide. More efficient cooling is also needed in transportation systems due to the rapidly increasing adoption of hybrid and electric vehicles that rely on complex electrical systems, including electric motors and batteries that produce significant amounts of heat. More efficient cooling of these electronic systems would translate to increased driving range and utility of the vehicles.
Attempts to cool modern microprocessors using conventional air-cooling techniques have resulted in cooling systems with large heat sinks and large, power-consuming fans. In a common arrangement, a large heat sink is mounted on the microprocessor, and a large fan is mounted within a housing of the computer such that the fan draws cool room air into the housing and forces warm air out of the housing. But fans can be noisy, especially when many computers are provided in a common location, such as a data center, and each computer has one or more fans operating at high speeds. In addition, each metallic heat sink adds mass and cost to the computer and places mechanical stress on the microprocessor to which it is mounted. This stress can damage the microprocessor during transportation and handling and can fatigue the electrical connections (e.g. soldered connections) that connect the microprocessor to the motherboard of the computer. Operating speeds of next generation microprocessors will continue to increase, as will heat fluxes associated therewith, where heat flux is defined as heat load per unit area. Conventional air-cooling systems will soon be incapable of efficiently and effectively cooling these next generation microprocessors.
Currently, the majority of computer systems in residential and commercial settings are cooled using forced air cooling systems in which room air is forced, by one or more fans, over a series of extended metal surfaces (i.e. fins) coupled to microchips or other electronic components, such as power supplies. Cooling fans typically operate at high speeds and can be quite noisy. As the air passes over the electronic components, the air, which is at a lower temperature than the surfaces of the electronic components, absorbs heat from the electronic components, thereby cooling the components. These air-cooled systems are inherently limited in terms of performance and efficiency due to the low volumetric heat capacity of air, which is much lower than the volumetric heat capacity of water and other liquids. Because air has such a low heat capacity, high air flow rates are required to ensure adequate cooling of even relatively small heat loads. For instance, an air flow rate of 5 to 10 cubic feet per minute (“cfm”) is needed to cool a 100-watt heat load. For heat sources such as microprocessors, which as mentioned above can easily produce more than 40 watts per square centimeter, very high volumetric air flow rates are required to remove heat from the microprocessor to prevent overheating and intervention by a thermal protection mechanism that will reduce the processor speed to decrease its temperature. For an installation of one rack of servers, which is commonly used in computer rooms of small businesses and schools, two air conditioning units sized for a typical U.S. home are required to cool the computer room. Typical data centers, which can have several hundred racks of servers, must be equipped with special computer room air conditioning (“CRAC”) units that are large and expensive and must be professionally installed, often requiring substantial physical modifications to the facility to accommodate the CRAC, including installation of custom air ducting.
Air-cooled systems are not only inefficient at removing heat from electronics, but they also cause the electronics they cool to operate less efficiently. A typical microprocessor operates less efficiently as its junction temperature increases. FIG. 64 shows a plot of CPU power consumption in watts versus junction temperature. The bottom curve shows static power consumption of the microprocessor and the top curves show total power consumption for switching speeds of 1.6 GHz and 2.4 GHz, respectively. Total power consumption includes both static power consumption and dynamic power consumption, which varies with switching frequency. As shown in FIG. 64, as the temperature of the microprocessor increases, it consumes more power to provide the same performance. In air-cooled systems, it is common for fully utilized microprocessors to operate at or near their maximum rated temperature, resulting in poor operating efficiency. In the example shown in FIG. 64, the microprocessor uses over 35% more power when operating at 370K than when operating at 320K.
Pumped liquid cooling systems have been used to provide improved thermal performance over conventional air-cooling systems. Pumped liquid cooling systems typically include a heat sink that is attached to the microprocessor or other circuit to be cooled, a liquid-to-air heat exchanger, a single phase liquid pump, and connection tubing. A thermally conductive liquid coolant is circulated through the system by the pump. As the liquid passes through channels in the heat sink, heat from the hot processor is transferred to the heat sink and to the cooler liquid. The heat sink is typically designed to maximize heat transfer by maximizing the surface area of the channels through which the liquid passes. For example, micro-channel heat sinks utilize very fine fin channels through which cooling fluid flows. The hot liquid exiting the heat sink is then circulated through the liquid-to-air heat exchanger before circulating back to the liquid pump for another cycle. Use of such closed liquid cooling systems is beginning to migrate from high performance computers to personal computers. However, even the best pumped liquid cooling systems are limited in their ability to maintain low device temperatures without the use of refrigeration and will not be able to satisfy the cooling demands of next-generation microprocessors. Without further innovation in the area of heat removal systems the development of next-generation microprocessors and other electronic devices will be hampered.
As noted above, liquid cooling systems commonly rely on flowing single-phase water through channels in finned heat sinks. The heat sinks are often indirectly coupled to a heat source via a metal base plate, thermal paste (e.g. solder thermal interface material (“STIM”) or polymer thermal interface material (“PTIM”)), and/or a direct bond adhesive. While this approach can be more effective than air-cooling, the intervening materials between the water and the heat source (e.g. microprocessor) induce significant thermal resistance, which reduces the overall efficiency of the cooling system. In addition to higher thermal resistance, the intervening materials add to the cost and time of manufacture and assembly, constitute additional points of failure, and create potential disposal issues. Finally, the intervening materials render the system unable to adapt to local hot spots on a heat source. Consequently, the entire liquid cooling system must be designed to accommodate the maximum anticipated heat load of one or more localized hot spots on the surface of the heat source, resulting in additional cost and complexity.
Further improvements have been made to liquid-cooled systems by using coolants other than water. Unlike water, dielectric coolants can be placed in direct contact with electronic devices and not harm them. Use of such dielectric coolants can eliminate a significant amount of thermal interface material from the system. However, some dielectric coolants have a lower heat capacity than water, so more aggressive cooling techniques may be required to achieve a desired performance.
Immersion cooling is an aggressive form of liquid cooling where an entire electronic device is submerged in a vat of dielectric coolant. Unfortunately, immersion cooling requires vats that are large, costly, and heavy, especially when filled with a dielectric coolant. Typically, a room must be specially engineered to accommodate an immersion cooling vat, and containment systems may need to be designed and installed in the room as a precaution against vat failure. Immersion cooling can require large volumes of costly dielectric coolants. Another downside of immersion cooling is that certain coolants may act as solvents and, over time, remove certain identifying information (e.g. printed serial numbers or model numbers) from electronic components on a motherboard, which can make servicing the computer more difficult.
Another liquid cooling approach involves atomized sprays, in which atomized liquid coolant is sprayed directly on a surface through air or vapor. As a result, small droplets impinge on the heated surface forming a thin film of liquid directly on the computer chips. Heat is then transferred from the heated surface to the liquid either by sensible heating of the bulk liquid or by boiling off of a fraction of the liquid (i.e. latent heating). Although high heat removal rates can be achieved, a limiting heat flux is reached when the bulk liquid warms (e.g. through sensible heating) to its boiling point at any location on the surface being cooled. At this location, the surface will eventually dry out, at which point the surface temperature will rise very quickly, and the device may be damaged. This method of heat removal is known as spray cooling or spray evaporative cooling and is a very efficient method of removing high heat fluxes from small surfaces. Unfortunately, the margin for error in spray cooling systems is very narrow and the onset of dry out is a constant concern that can have costly consequences.
Spray cooling is limited by several factors. First, spray cooling requires a significant working volume to enable atomized sprays to form. Second, atomizing the liquid requires a significant amount of pressure upstream of the atomizer to generate an appropriate pressure drop at the atomizer-air interface to enable atomized sprays to form. Maintaining this amount of pressure within the system consumes a significant amount of energy. Third, high flow rates of atomized sprays are required to prevent dry out or critical heat flux from being attained. Critical heat flux is a condition where evaporation of coolant from the surface to be cooled 12 prevents atomized liquid from reaching the surface and cooling the surface. In the end, it has proven difficult to design a practical and compact spray cooling system, despite the large amount of time and effort that has been expended to do so.
Another liquid cooling approach involves direct jet impingement, where streams of liquid are projected through a liquid medium and impinge directly on a surface to be cooled 12. While impinging jets are known to have notable heat transfer performance, impinging jet systems have problems of scalability. To achieve high heat transfer over a large area, arrays of jets must be used. The use of arrays in conventional direct jet impingement systems, however, is problematic. Opposing surface flows of fluid from neighboring jet streams emitted from the array of jets can induce stagnant regions on the surface to be cooled 12. Stagnation regions prevent cooler fluid from mixing with warmer fluid in the stagnation regions, leading to bubble growth and dry out at the surface being cooled as the warmer fluid experiences phase change. Thus, the interaction of jet streams can lead to inefficient cooling caused by liquid build-up on the heated surface, creating regions of poor heat transfer and non-uniform heat transfer across the surface being cooled. In the regions of poor heat transfer, the surface temperatures can rise significantly above the average surface temperature, causing the surface temperature in these regions to “run away,” leading to catastrophic failure of the device being cooled.
Conventional jet impingement systems use nozzles that are part of a large, flat nozzle plate. As fluid from jet streams impinging on the surface being cooled flow outward from the center of the surface, the fluid can have sufficient momentum to completely deflect the outermost jets, preventing them from impinging on the heated surface near the edge of the surface. As a result of these factors, conventional impinging jet systems are limited in size. In addition, existing nozzle plates can be costly and complex to manufacture.
In view of the foregoing discussion, efficient, scalable methods and apparatuses are needed for cooling surfaces of devices, such as next-generation microprocessors and electronic circuitry, capable of producing high heat fluxes.